1. Field of the Disclosure
Embodiments disclosed herein relate generally to the preparation of dimethyl ether by catalytic reaction of methanol with itself and the concurrent distillation and separation of the product and reactants. More particularity the invention relates to a process for producing essentially pure dimethyl ether and water.
2. Background
The preparation of ethers by the dehydration of alcohols using an acid is known, e.g.,R—OH+H2SO4→R—O—SO2—OH+H2OR—O—SO2—OH+OH—R→R—O—R+H2SO4 
U.S. Pat. No. 3,267,156 discloses that acidic cation exchange resins effectively catalyze the selective dehydration of alcohols to ethers as exemplified by the production of diisopropyl ether.
U.S. Pat. No. 3,931,349 discloses a process to convert methanol to gasoline boiling components. In a first step, a methanol feed is vaporized and converted to a mixture of dimethyl ether, methanol, and water by contact with a catalyst, such as gamma alumina. The exothermic alcohol dehydration reaction increases the temperature of the resulting stream such that contact of the stream with a ZSM-5 zeolite results in the formation of gasoline boiling range aromatics. The dehydration reaction is indicated as releasing about 750 BTU per pound of methanol which may cause large temperature increases and may result in high catalyst aging rates.
U.S. Pat. No. 5,037,511 discloses a process to produce pure dimethyl ether by catalytic dehydration of methanol at a temperature of 140-500° C. and a pressure of 1-50 bar followed by distillation. The dehydration product is fed to a distillation column to separate the dimethyl ether from water, unreacted alcohol, and reaction byproducts where the dimethyl ether is withdrawn from the column at one or more trays that are at least one tray above the bottom of the column, characterized in that (a) liquid and/or gaseous fraction(s) containing reaction by-products are withdrawn at least 3 trays below the lowest tray from which pure dimethyl ether is withdrawn, and (b) the tray(s) from which pure dimethyl ether is withdrawn, is at least 8 trays above the highest feed tray.
U.S. Pat. No. 5,316,627 discloses a process for the production of odorless dimethyl ether. An alcohol dehydration product stream (i.e., a crude dimethyl ether stream), containing methanol, dimethyl ether, and water, is fed to a single distillation column where a dimethyl ether draw is subsequently treated with an insoluble acid ion exchange resin to remove impurities.
U.S. Pat. No. 5,684,213, incorporated herein by reference, discloses a process for the production of dialkyl ethers in a distillation column reactor in the presence of added hydrogen. While various ranges of conditions are disclosed for ether production, the conditions specifically disclosed for the production of dimethyl ether are pressures ranging up to 600 psig and a catalyst zone temperature of 350-400° C. (662-752° F.). Also disclosed is that the overhead product stream may be subsequently fractionated to result in 99.9+% pure dimethyl ether.
Although described as benefiting the process, addition of hydrogen to the distillation column reactor may add to the operating and capital costs of the process. For example, hydrogen may result in unwanted by-products, may require the use of compressors and higher pressure-rated equipment, and may result in increased piece count, among others. Additionally, each of the above-described processes describes the dehydration of alcohol to form a crude ether product stream which is subsequently separated to form a pure ether product.
Accordingly, there exists a need for a catalytic distillation process for the production of substantially pure dialkyl ethers which does not require the use of hydrogen. Additionally, there also exists a need for processes which may eliminate the need for downstream separation, treatment, or purification of the resulting dehydration reaction products.